Transition from the Tetragonal to Cubic Phase of Organohalide Perovskite: The Role of Chlorine in Crystal Formation of CH₃NH₃PbI₃ on TiO₂ Substrates

The role of chlorine in the superior electronic property and photovoltaic performance of CH₃NH₃PbI_(3-x)Cl_x perovskite has attracted recent research attention. Here, we study the impact of chlorine in the perspective of the crystal structure of the perovskite layer, which can provide important understanding of its excellent charge mobility and extended lifetimes. In particular, we find that in the presence of chlorine (PbCl₂ or CH₃NH₃Cl), when CH₃NH₃PbI₃ films are deposited on a TiO₂ mesoporous layer instead of a planar TiO₂ substrate, a stable cubic phase rather than the commonly observed tetragonal phase is formed in CH₃NH₃PbI₃ perovskite at room temperature. The relative peak intensity of two major facets of cubic CH₃NH₃PbI₃ crystals, (100)_C and (200)_C facets, can also be easily tuned, depending on the film thickness. Furthermore, compared with pristine CH₃NH₃PbI₃ perovskite films, in the presence of chlorine, CH₃NH₃PbI₃ crystals grown on planar substrates exhibit strong preferred orientations on (110)_T and (220)_T facets.